1. Field of the Invention
The present invention relates to a liquid crystal driving device for driving a display device such as a liquid crystal display projector or the like.
2. Description of the Related Art
First, the system construction of a conventional liquid crystal driving device and the construction and function of each part will be briefly described.
FIG. 1 is a block diagram showing an example of the overall system construction of a conventional liquid crystal driving device. Reference numeral 10 represents an RGB signal processing circuit, reference numerals 20R, 20G, 20B represent RGB drivers, reference numerals 30R, 30G, 30B represent LCD panels, and reference numeral 40 represents a timing generator.
FIG. 1 shows a three-plate type projector in which an individual panel is used for each of R, G, B colors, and three LCD panels 30R, 30G, 30B are disposed.
The RGB signal processing circuit 10 has a function of performing pre-processing of an input video signal in order to drive the video signals (R, G, B signals) and perform signal processing such as cut-off adjustment, etc.
The RGB driver 20R, 20G, 20B represents a signal processing circuit having a function of perform processing such as clamp, gamma, amplitude, bias adjustment, etc. on the R, G, B signals respectively, and in this case paralleling processing is performed.
The LCD panels 3OR, 30G, 30B are driven by video signals of colors R, G, B respectively, and control the light amount from a light source (not shown).
Timing signals which are required for the RGB drivers 20R, 20G, 20B and the LCD panels 30R, 30G, 30B are generated by the timing generator 40.
FIG. 2 is a functional block diagram showing an example of the detailed construction of the RGB driver shown in FIG. 1. Reference numeral 21 represents a gamma circuit, reference numeral 22 represents a gain/bias adjustment circuit, and reference numerals 23 to 27 represent a sample hold circuit.
Each of the RGB drivers 20R, 20G, 20B is constructed as shown in FIG. 2.
The sample hold circuits 23 to 27 parallel the signal passed through the gamma circuit 21 and the gain/bias adjustment circuit 22 on the basis of three-phase sample hold pulses which are different in phase.
Therefore, three-phase pulse signals of SH1, SH2, SH3 are generated from a clock serving as a reference in the timing generator 40, and supplied to the respective RGB drivers 20R, 20G, 20B.
Of these three-phase pulse signals, SH3 is a pulse for re-sampling, and used to sample and hold the output from the sample hold circuits 23, 24 again.
With respect to the output S3, the sampling and holding is performed only once by the re-sampling pulse SH3.
FIG. 3 is a functional block diagram showing an example of the detailed construction of the peripheral portion of the LCD panel shown in FIG. 1. Reference numeral 30 represents an LCD panel, reference numeral 31 represents an H shift register, reference numeral 32 represents a V shift register, and SW1 to SW3 represent first to third switches.
The LCD panel 30R, 30G, 30B is constructed as show in FIG. 3.
The liquid crystal display device shown in FIG. 1 is applied to a case where a plural simultaneous sampling is performed as a method of driving the LCD display panel.
According to this LCD display panel driving method, there is obtained an advantage that reduction of the signal band and reduction of the clock frequency of the shift register can be performed by subjecting the video signals of the respective colors R, G, B to paralleling processing with the RGB drivers 20R, 20G, 20B.
A method of filtering video signals has been known as a countermeasure of removing high-frequency components contained in the video signals and high-frequency components generated in the processing stage of the video signals in the driving operation of the liquid crystal display device shown in FIGS. 1 to 3.
Here, the disturbance wave due to the conventional filtering processing of the video signals will be described.
FIGS. 4A and 4B show an example of waveform which disturbs a picture on a frame and a displayed image in the liquid crystal driving device. FIG. 4A represents the relationship between the waveform of the video signal and the sampling positions, FIG. 4B represents sampling waveform, and FIG. 4C represents a display result.
In FIGS. 4A and 4B, pixels are disposed in a staggered arrangement, and the sampling timing is varied between odd-number lines and even-number lines.
In FIG. 4A, an upward arrow represents a sampling timing on the LCD panel (30R, 30G, 30B).
That is, in the case of FIGS. 4A and 4B, the relationship between the waveform of the video signal and the sampling position are set so that the sampling is performed at different positions (timing) between the odd-numbered lines and the even-numbered lines as indicated by the upward arrow of FIG. 4A.
Therefore, the sample waveform of each of the odd-numbered line and the even-numbered line as shown in FIG. 4B becomes sample waveform having L level when the video signal is in L level, and also becomes sample waveform having H level when the video signal is in H level.
Further, when the video signal have an intermediate level between the L level and the H level, the sample waveform having the intermediate level can be obtained, for example, like the even-numbered line.
When the LCD panel is driven with the output of the sample waveform shown in FIG. 4B, a frame as shown in FIG. 4C is displayed.
Summarizing the above operation (phenomenon), in the conventional liquid crystal driving device, the pixel and the sampling timing are in one-to-one correspondence with each other, so that the sample waveform of the odd-numbered line and the even-numbered line as shown in FIG. 4B is obtained, and a notch-emphasized pattern due to the structure of the dot arrangement is displayed on the frame as show in FIG. 4C (for example, Japanese Unexamined Patent Application No. Hei-7-261148).
Such a notched pattern as described above is visualized as an obstacle to pictures, and thus the quality of the display image is lowered.
In the case of the conventional liquid crystal driving device, when the method of filtering the video signal is adopted to remove the high-frequency components contained in the video signal and the high-frequency components generated in the processing stage of the video signal, there occurs a problem that such a side effect as greatly lowers sharpness is strong.
Further, when the filter is constructed by an analog circuit, there is also a problem that it is liable to suffer an effect due to dispersion of elements.
The present invention has an object to provide a liquid crystal driving device which enables a display with high image quality by effectively removing high-frequency components which are contained in video signals to disturb pictures on a display frame and cause image quality to be lowered and are generated in a processing stage of the video signals.
In order to attain the above object, a liquid crystal driving device according to the present invention is characterized in that a timing circuit for generating a sampling pulse is constructed by a PLL circuit comprising a voltage control type oscillator, a phase comparator for synchronizing an oscillation frequency with an input signal, and a filter for smoothening a comparison result, a phase shifter for shifting the clock phase periodically to vary the sampling phase and vary the phase relationship between a signal and a pixel periodically, and input means for an alternating signal which comprises a periodic wave and is modulated every line or every field by a pulse wave, and that a fixed pixel is restructured by periodically varying the phase relationship between the alternating signal and the pixel to provide a visual filtering effect.
Accordingly, firstly, the high frequency components which disturb pictures on the frame and cause reduction in image quality in the conventional liquid driving device can be removed.
Secondly, the high frequency components which occur due to sampling or pixel structure and have been difficult to be removed by the conventional technique can be removed.
Thirdly, a pixel position to be displayed is controlled in place of an operation of video signals, so that a spatial frequency filter can be implemented at a low price.
Fourthly, since no signal processing filter is used, there can be obtained may excellent effects such as an effect that deterioration of sharpness is little, etc.
Further, in the liquid crystal driving device, the timing generator is provided with a variable phase shifter which can control the phase of a sampling block output from the PLL circuit in the timing circuit block, and input means for an alternating voltage signal which periodically varies the phase of the variable phase shifter, thereby periodically varying the phase relationship between the signal and the pixel.
The same effect of the liquid crystal driving device described above can be obtained even when the timing generator thus constructed is used.
Further, in the liquid crystal driving device described above, the timing generator is provided with a variable phase shifter which can control the phase of the clock in the PLL circuit in a timing circuit block, and input means for an alternating voltage signal which periodically varies the phase of the variable phase shifter, thereby periodically varying the phase relationship between the signal and the pixel.
The same effect of the liquid crystal driving device described above can be obtained even when the timing generator thus constructed is used.